1. Field of the Invention
This invention relates to superconducting oxide articles having improved characteristics for alternating current operation and to a method for manufacturing them. In particular, the invention relates to superconducting composite wires including tightly twisted filaments of a superconducting oxide material and to a manufacturing method which includes torsional deformation.
2. Background of the Invention
The geometries in which high-performance superconducting oxide composites may be successfully fabricated are constrained by the necessity of texturing the material to achieve adequate critical current density. The current-carrying capacity of a superconducting oxide composite depends significantly on the degree of crystallographic alignment and intergrain bonding of the oxide grains, together known as xe2x80x9ctexturingxe2x80x9d, induced during the composite manufacturing operation.
Known processing methods for texturing superconducting oxide composite articles include various forms of heat treatment as well as longitudinal deformation. Certain superconducting oxide grains can be oriented along the direction of an applied strain, a phenomenon known as deformation-induced texturing (DIT). Longitudinal deformation techniques like pressing, drawing and rolling have been used to induce grain alignment of the oxide superconductor c-axis perpendicular to the plane or direction of elongation. Heat treatment under conditions which at least partially melt and regrow desired superconducting phases may promote texturing by enhancing the anisotropic growth of the superconducting grains, a phenomenon known as reaction-induced texturing (RIT).
However, not all texturing methods are equally applicable to, or effective for, all superconducting oxides. Most of these materials have very few known effective texturing mechanisms. For example, known techniques for texturing the two-layer and three-layer phases the bismuth-strontium-calcium-copper-oxide family of superconductors (BSCCO 2212 and BSCCO 2223, respectively) are described in Tenbrink, Wilhelm, Heine and Krauth, Development of Technical High-Tc Superconductor Wires and Tapes, Paper MF-1, Applied Superconductivity Conference, Chicago(Aug. 23-28, 1992), and Motowidlo, Galinski, Hoehn, Jr. and Haidar, Mechanical and Electrical Properties of BSCCO Multifilament Tape Conductors, paper presented at Materials research Society Meeting, Apr. 12-15, 1993. Techniques for manufacturing multifilamentary articles with sufficient texturing to provide acceptable critical current densities from BSCCO 2223 are presently limited to the production of highly aspected forms such as tapes.
The effectiveness of a particular DIT technique will depend on how closely the applied strain vectors correspond to the slip planes in the superconducting oxide. Thus, superconducting oxides such as the BSCCO family, which have a micaceous structure characterized by highly anisotropic preferred cleavage planes and slip systems, are known to be most effectively DIT textured by non-axisymmetric techniques such as pressing and rolling, which create highly aspected (greater than about 5:1) forms. For perovskite structures like the 123 phase of the yttrium-barium-copper-oxide (YBCO) family, which lack preferred cleavage planes and slip systems, longitudinal deformation is generally less effective in improving critical current density and the differences in texturing obtainable by axisymmetric and non-axisymmetric techniques are less pronounced.
Materials which exhibit peritectic melting can be effectively textured in a variety of geometries by melt textured growth, an RIT technique. Peritectic decomposition and the reformation of the oxide superconductor from the liquid+(other) solid phase is the basis for melt textured growth of the two-layer phases of the bismuth-strontium-calcium-copper-oxide family of superconductors(BSCCO-2212) in round wire and tape forms, as described, for example, in Kase et al, IEEE TransMag 27(2), 1254(March 1991). Because 2212 totally melts and reforms during melt-textured growth, the texturing induced by deformation prior to the melting will not influence the final structure.
However, some of the most promising superconducting oxides, such as BSCCO 2223, cannot be effectively textured by the melt-textured growth technique. Instead of peritectic melting, BSCCO 2223 exhibits irreversible melting in that solid 2223 does not form directly from a liquid of 2223 composition. RIT techniques applicable to BSCCO 2223 have been described, for example in U.S. patent applications Ser. No. 08/041,822 filed Apr. 1, 1993, entitled xe2x80x9cImproved Processing for Oxide Superconductors,xe2x80x9d and Ser. No. 08/198,912 filed Feb. 17, 1994, entitled xe2x80x9cImproved Processing of Oxide Superconductorsxe2x80x9d. The basis of such techniques is some type of partial melting, such as eutectic melting, solid solution melting or formation of non-equilibrium liquids, in which the oxide superconductor coexists with a liquid phase rather than being totally decomposed. However, such techniques are inherently more dependent on the geometry of the initial phase than melt-textured growth, and texturing induced by deformation prior to the partial melting will have a significant impact on the texturing of the final product. The RIT technique described in U.S. patent application Ser. No. 08/041,822 cited above, for example, has been observed to provide the greatest improvement in the Jc""s of BSCCO 2223 samples when used in combination with a highly non-axisymmetric DIT technique, rolling. In short, superconducting oxides with irreversible melting characteristics such as BSCCO 2223 can be adequately textured by known techniques in highly aspected forms such as tapes, but scalable methods for manufacturing round wires and other low aspect ratio composite multifilamentary articles with sufficient texturing to provide acceptable critical current densities are not presently available.
This limitation has considerable commercial significance. Many of the superconductor applications that have the greatest potential for energy conservation involve operating the superconductor in the presence of an AC magnetic field, or require that the superconductor carry an AC current. In the presence of time-varying magnetic fields or currents, there are a variety of mechanisms that give rise to energy dissipation, hereafter called AC losses, even in superconductors. Thus, the superconductor geometry must be selected to reduce AC losses, in order to preserve the intrinsic advantage of superconductors, the absence of DC electrical resistance. The physics governing AC losses in low temperature superconducting composite materials have been described and analyzed, c.f. Wilson, Superconducting Magnets, Ch 8(1983, 1990), and round, multifilamentary composite geometries with twisted, low aspect ratio superconducting filaments have been demonstrated to have significantly better AC loss characteristics than highly aspected, untwisted or monofilamentary forms. To minimize hysteretic losses, the superconductor must preferably be subdivided into many small filaments that are dimensionally uniform and discrete along the length of the conductor. Low aspect ratio filaments (about 4:1 or less) will have lower hysteretic losses in all but unidirectional magnetic fields, so these filament dimensions are generally preferred. To minimize eddy current losses, the matrix resistivity must preferably be increased and the twist pitch of the filaments must preferably be tightened, i.e., reduced. Electrical properties which are radially homogeneous, that is homogenous around the circumference at any given cross-sectional radius, and are also homogeneous along the length of the article are preferred to assure uniform performance. In order to achieve this, it is desirable for both the distribution of the filaments and the degree of texturing to be substantially uniform along these dimensions. Homogeneous properties along the cross-sectional diameter are not required. In fact, for many AC applications electrical performance characteristics which are much poorer at the center of the cross-section than near the circumference may be preferred to reduce longitudinal eddy currents. In general, the primary sources of AC loss in low temperature superconducting composites, hysteretic loss within the superconducting filament(s),and eddy current loss in the noble metal matrix enhanced by coupling between superconducting filament(s), would be expected to operate in superconducting oxide composites with similar geometries. However, no conventional DIT or RIT texturing technique provides increased texture with increasing distance from the center of the article. In fact, the non-axisymmetric longitudinal deformations conventionally used to texture BSCCO 2223 provide a texturing distribution which is radially non-uniform and highest at the center of the article, an unfortunate distribution for AC loss minimization. The availability of round, multifilamentary BSCCO-2223 composites with suitably textured, low aspect ratio, tightly twisted filaments would be highly desirable for AC applications, and would substantially increase the prospects for near-term commercialization of this material.
Thus, an object of the invention is to provide a novel method of texturing multifilamentary articles comprising superconducting oxides and their precursors to provide improved AC loss characteristics without compromising critical current density.
Another object of the invention is to provide a high performance multifilamentary superconducting article wherein the degree of texturing is substantially radially homogeneous at any given cross-section of the article and varies substantially in proportion to the radial distance from the center of the article cross-section.
Another object of this invention is to provide a method of texturing multifilamentary superconducting composite articles with low aspect ratio filaments which is suitable for use with superconducting oxides which melt irreversibly.
Another object of the invention is to provide round twisted multifilamentary BSCCO 2223 composite conductors having high current densities and robust mechanical properties, and a method for producing them.
In one aspect, the invention provides a method of texturing a multifilamentary article having filaments comprising a desired oxide superconductor or its precursors by torsionally deforming the article. By xe2x80x9ctexturingxe2x80x9d, as that term is used herein, is meant inducing crystallographic alignment and intergrain bonding of the grains of a desired superconducting oxide or its precursors. Desired oxide superconductors or precursors with micaceous or semi-micaceous structures are preferred. The texturing is induced by applying a torsional strain which is at least about 0.3 and preferably at least about 0.6 at the surface of the article, but less than the strain which would cause failure of the composite. By failure of the composite, as this term is used herein, is meant breakdown of composite material, substantial loss of dimensional uniformity, or failure of the bonds between the filaments and the matrix material. It is preferred that the total strain applied should be sufficient to provide a twist pitch tighter than 5 times the diameter of the article at the time the strain is applied, and twist pitches in the range of 1 to 5 times the diameter of the article are most preferred. If tighter twist pitches are desired, the article may be heat treated or annealed and the torsional texturing step may be repeated as many times as necessary to obtain the desired twist pitch. Strains of this magnitude will provide twist pitches in the range of about 1.00 to 0.01 inch (25 to 0.25 mm), that is, about 1.00 to 80 twists per inch, for articles with diameters in the preferred range of 0.005 inch to 0.12 inch (0.13 to 3 mm) at the time the torsional strain is applied.
Preferably, the article is textured by applying a sequence of separate texturing steps including both torsional strains and conventional texturing techniques selected from the set including axisymmetric longitudinal deformation, isostatic pressing or roller straightening and RIT techniques dependent on partial melting. Any conventional technique which does not eliminate prior texturing or substantially change the aspect ratio of the article may be used. Preferably, at least one torsional deformation step precedes the first conventional texturing step. In the most preferred embodiment, one or more successive iterations of torsional and axisymmetric longitudinal deformation may be used, and heat treatments sufficient to produce RIT, phase transformation of precursors, or both, in the filament material are most preferably done after each deformation step except the last.
In another aspect, the invention provides a method for manufacturing a high performance multifilamentary superconducting article, such as a composite wire, having a plurality of low aspect ratio twisted filaments, each comprising a textured desired superconducting oxide material and, preferably, a noble metal matrix in intimate contact with said filaments. The process comprises the steps of forming a composite comprising a plurality of filaments of material comprising a desired superconducting oxide or its precursors, and preferably selected to have a micaceous or semi-micaceous structure, torsionally deforming the composite as described above in order to twist the filaments and texture the material comprised therein, and thermomechanically processing the composite at conditions sufficient to produce at least one of the effects of RIT, crack healing and, if a precursor material remains, phase transformation in the filament materials. Preferably, as described above, the article is textured by applying a sequence of separate texturing steps including both torsional strains and conventional texturing techniques selected from the set including axisymmetric longitudinal deformation, isostatic pressing or roller straightening and RIT techniques dependent on partial melting.
In yet another aspect, the invention provides a high performance multifilamentary superconducting article, having a plurality of low aspect ratio (preferably less than about 4:1) twisted filaments, each comprising a textured desired superconducting oxide material, and, preferably, a noble metal matrix in intimate contact with said filaments. It is preferred that the desired oxide superconductor or its precursors be selected to have a micaceous or semi-micaceous structure. Micaceous structures are characterized by highly anisotropic preferred cleavage planes and slip systems, and semi-micaceous structures are characterized by a highly anisotropic, plate-like structure but poorly defined cleavage planes and slip systems. This invention is particularly well adapted for use with micaceous desired superconducting oxides which melt irreversibly, such as BSCCO 2223. By xe2x80x9cdesired oxide superconductorxe2x80x9d, as that term is used herein, is meant the oxide superconductor intended for eventual use in the finished article. Typically, the desired oxide superconductor is selected for its superior electrical properties, such as high critical temperature or critical current density. By xe2x80x9cprecursorxe2x80x9d, as that term is used herein, is meant any material that can be converted to an oxide superconductor upon application of a suitable heat treatment. By xe2x80x9cnoble metalxe2x80x9d, as that term is used herein, is meant a metal which is substantially non-reactive with respect to oxide superconductors and precursors and to oxygen under the expected conditions (temperature, pressure, atmosphere) of manufacture and use. By xe2x80x9cmatrixxe2x80x9d as that term is used herein, is meant a material or homogeneous mixture of materials which supports or binds a substance disposed within or around the matrix. xe2x80x9cAlloyxe2x80x9d is used herein to mean an intimate mixture of substantially metallic phases or a solid solution of two or more elements. Silver and alloys comprising silver are the preferred noble metal matrix materials, but other metals and alloys may be used.
In yet another aspect, the invention provides an elongated high performance multifilamentary superconducting article with improved AC loss characteristics. The article comprises a plurality of twisted filaments with substantially uniform dimensions along the length of the article, each filament at a given radial distance from the center of the article cross-section having a uniform dimension parallel to the radius and comprising a textured desired superconducting oxide material wherein the degree of texturing increases with the radial distance from the center of the article cross-section, and is substantially radially homogeneous at any given cross-section of the article. Round wires and other low aspect ratio multifilamentary articles are preferred forms. The invention is not dependent on the melting characteristics of the desired superconducting oxide. When used in connection with desired superconducting oxides which melt irreversibly, it provides multifilamentary articles that exhibit high DC performance characteristics and AC performance markedly superior to any currently available for these materials. In a preferred embodiment, the desired superconducting oxide material is BSCCO 2223.